Mark, G. (2001) Nd isotope and petrogenetic constraints for the origin of the Mount Angelay igneous complex: implications for the origin of intrusions in the Cloncurry district, NE Australia. Precambrian Research, 105 (1). pp. 17-35.

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Abstract

The Mesoproterozoic Mount Angelay igneous complex contained intrusions that were emplaced into amphibolite facies metasedimentary rocks during two periods of ~1550 and post-1540 Ma magmatism. Sm–Nd isotopic analysis together with mineralogical and chemical considerations suggest that the intrusions were produced from a Paleoproterozoic crustal source with a T2 model age ~2200 Ma. On geochemical and petrological grounds, the ~1550 Ma trondhjemitic intrusions are interpreted to have been produced by melting of amphibolite under garnet-stable conditions (>8–10 kbar). The late-syn to post-peak metamorphic timing of these intrusions suggested that they were associated with the tectono-thermal event that produced regional peak metamorphic mineral assemblages. The post-1540 Ma intrusions are K-rich and consist of two groups of synchronously emplaced intrusions, (1) a high-K monzodiorite and monzogranite suite that range between 51 and 77 wt.% SiO2; and (2) a high-K, Na-enriched hornblende monzonite. The chemistry and mineralogy of these intrusions suggested that they were derived via plagioclase-stable and garnet-unstable melting (<8–10 kbar). The high-K monzodiorite and monzogranite are interpreted to have formed from a plagioclase-bearing source that contained abundant K-feldspar, biotite and/or amphibole. These intrusions are relatively enriched in K, Ca, LREE, Ba, Sr, Zr, Cl and F, and depleted in Na2O, P2O5, Cr, V and Zn compared with slightly younger high-K monzonite, which is interpreted to have formed via one of two mechanisms, (1) melting of a low-K amphibole- and plagioclase-rich source; or (2) melting of residual material that produces a potassic and incompatible element-rich melt. These magmas likely contained mantle-derived material, particularly the K-rich intrusions of mafic composition. The heat required for the production of post-1540 Ma intrusions appears to have been generated by the intrusion of high-T, mantle-derived, mafic material into the crust (~25–30 km; ~8–10 kbar). This model is consistent with the synchronous emplacement of mafic and felsic magma and the lack of a consanguineous regional metamorphic association, and suggests high-T, high-degree partial melting in localised pockets within fertile source regions in the crust. An increase in Sm–Nd model source age and decrease in εNd with increasing SiO2 in the K-rich intrusions suggests the incorporation of juvenile material in the more mafic rocks. The origin of this component is unknown, but it may represent either the incorporation of mantle-derived material during melting, or the partial melting of crust with a younger mafic component. On a district scale, the >30 million year period over which the K-rich post-1540 Ma intrusions were emplaced suggested that mantle-derived material continued to be injected into the crust. A mantle component to these rocks, and the global distribution of Proterozoic intrusions with similar geochemical affinities, strongly suggests a world-wide period of mantle-induced crustal melting at that time. The dominant Paleoproterozoic isotopic composition of most of these intrusions suggests melting of similarly composed and matured source rocks.

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